Non-linear resistor



March 24, 1970 MlcHlo MATsUoKA 3,503,029

NON-LINEAR REsIsToR Filed April 19, 1968 United States Patent Oihce 3,503,029 Patented Mar. 24, 1970 ABSTRACT F THE DISCLOSURE A non-linear resistor comprising a Sintered wafer of zinc oxide having opposite surfaces one of which has an electrical resistance higher than that of another surface and two electrodes applied to said opposite Surfaces, at least one of said two electrodes being in a non-ohrnic contact with said one surface having a higher electrical resistance.

This invention relates to non-linear resistors having non-ohmic resistance and more particularly to varistors comprising zinc oxide and silver electrodes applied thereto.

Various non-linear resistors such as silicon carbide varistors, selenium or cuprous oxide rectifiers and germanium or silicon p-n junction diodes, are known.

The electrical characteristics of such a non-linear resistor i are expressed by the relation:

V n I (c) where V is the voltage across the resistor, I is the current iiowing through the resistor, C is a constant equivalent to the voltage at a given current and exponent n is a numerical value greater than l. The valve of n is calculated by the following equation:

where V1 and V2 are the voltages at given currents I1 and I2, respectively. Conveniently, I1 and I2 are l0 ma. and 100 ma., respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the degree to which the resistors depart from ohmic characteristics.

Silicon carbide varistors are most widely used as nonlinear resistors and are manufactured by mixing line particles of silicon carbide with Water, ceramic binder and/or conductive material such as graphite or metal powder, pressing the mixture in a mold to the desired shape, and then drying and firing the pressed body in air or non-oxidizing atmosphere. Silicon carbide varistors with conductive materials are characterized by a low electirc resistance, i.e. a low value of C and a low value of n whereas silicon carbide varistors without conductive materials have a high electric resistance, i.e. a high value of C and a high value of n. It has been diilicult to manufacture `silicon carbide varistors characterized by a high n and a low C. For example, silicon carbide varistors with graphite have been known to exhibit nl values from 2.5 to 3.3 and C-values from 6 to 13 at a given current of 100 ma., and silicon carbide varistors without graphite show n-values from 4 to 7 and C-values from 30 to 800 at a given current of ma. with respect to a given size of varistor, e.g. mm. in diameter and 1 mm. in thickness.

Conventional rectiiiers comprising selenium or cuprous oxide have an n-value less than 3 and a C-value of 5 to 10 at a given current of 100 ma. with respect to a specimen size of 20 mm. in diameter. In this case, the thickness of sample does not affect the C-value.

A germanium or silicon p-n junction resistor has an extremely high value of n but its C-value is constant, e.g. of the order of 0.3 or 0.7 at a given current of ma. because its diffusion voltage in the V-I characteristics is constant and cannot be changed remarkably. It is necessary for obtaining a desirable C-value to combine several diodes in series and/or in parallel. Another disadvantage of such diodes is the complicated steps involved in their manufacture, with resultant high cost. As a practical matter, the use of diode resistors is not widespread at present in view of their high cost even though they may have a high value of n.

VArr object of this invention is to provide a non-linear resistor having'a high value of n and a low value of C.

A further object of this invention is to provide a non-linear resistor capable of being made by a simple manufacturing method which results in a low cost.

A further object of this invention is to provide a nonlinear resistor characterized by a high stability to temperature, humidity and electric load.

Another object of this invention is to provide a nonlinear resistor, the C-value of which can be controlled.

These and other objects of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single figure is a partly cross-sectional view through a non-linear resistor in accordance with the invention.

Before proceeding with a detailed description of the non-linear resistors contemplated by the invention, their construction will be described with reference to the aforesaid figure of drawing wherein reference character 10 designates, as a whole, a non-linear resistor comprising a Sintered wafer 1 of zinc oxide having two opposite surfaces 7 and 8. One of the said surfaces 7 or 8 has an electrical resistance higher than that of the other surface. For example, surface 7 has an electrical resistance higher than that of surface 8.

Said zinc oxide referred to herein should not be limited to pure zinc oxide and is defined as pure zinc oxide and zinc oxide incorporated with additives contemplated by the present invention yas described hereinafter.

Sintered wafer 1 is prepared in a manner hereinafter set forth, and is provided with a pair of electrodes 2 and 3 having specified compositions and severally applied in a suitable manner hereinafter set forth, on the two opposite surfaces 7 and 8 of the wafer.

The wafer 1 is a sintered plate having any one of various shapes such as circular, square, rectangular, etc. Wire leads 5 and 6 are attached conductively to the electrodes 2 and 3, respectively, by a connection means 4 (solder or the like).

According to the invention, a superior non-linear resistor comprising a sintered wafer of zinc oxide is achieved when an electrode 2 in non-ohrnic contact is applied to said one surface 7 having a higher electrical resistance than said other surface 8 and another electrode 3 in ohmic contact is applied to said other surface 8 having a lower electrical resistance. Such a non-linear resistor can be used as an excellent voltage regulating means in DC or AC current and produces especially superior performance when it is used in D.C. current in such a way that the one surface having the higher resistance is connected to an electrically positive side.

A higher electrical resistance of the surface referred to herein can be determined by the following method: A Sintered disc is thoroughly covered, at both surfaces, with In electrodes in ohmic contact and is subjected to an electrical resistance measurement across both In electrodes by a Wheatstone bridge. Subsequently, the disc is lapped at one surface for removal of surface layer in a thickness more than 0.2 mm. and then is provided with an In electrode at such surface. The electrical resistance measurement is made with the resultant disc. A comparison between the electrical resistance of the unlapped disc and that of the lapped disc decides how high the electrical resistance of the surface layer is.

The non-linear resistor according to the invention can be modified in such a way that two electrodes innonohmic contact are applied to one surface having a high electrical resistance and to another surface having a low electrical resistance, respectively. Such a modified nonlinear resistor can be also used as an excellent voltage regulating means in DC or AC current.

According to the present invention, sintered wafer 1 consists essentially of, as an active ingredient, zinc oxide (ZnO). It is advantageous that said zinc oxide have incorporated therein a minor proportion of aluminum oxide (A1203), iron oxide (Fe203), bismuth oxide (Bi203) magnesium oxide (MgO), calcium oxide (CaO), nickel oxide (NiO), cobalt oxide (C0203), niobium oxide (Nb205), tantalum oxide (Ta2O5), zirconium oxide (ZrO2), tungsten oxide (W03), cadmium oxide (CdO), or chromium oxide (Cr203), as additive.

Any suitable and available method can be applied for making one surface of the aforesaid opposite two surfaces different in electrical resistance from the other surface.

It has been discovered according to the invention that a surface layer having a high electrical resistance is formed on the surface of a sintered wafer of zinc oxide sintered in an oxidizing atmosphere such as oxygen or air and cooled in the oxidizing atmosphere. Upon the basis of such discovery, the surface resistivity of the sintered wafer of zinc oxide can be lowered by removing a surface layer of a thickness more than 0.2 mm. The removal of a surface layer at one surface can be effected by a mechanical lapping method using a silicon carbide abrasive or alumina abrasive or a chemical etching method in accordance with the Well known prior art. Operable etching reagent is mineral acid, alkali hydroxide or ammonium chloride.

The aforesaid electrode in non-ohmic contact comprises a silver electrode prepared by firing-on a silver electrode paint applied to the surface of the sintered wafer of zinc oxide at 100 to 850 C. in an oxidizing atmosphere such as air or oxygen and then being cooled to room temperature in the oxidizing atmosphere. Operable and advanweight percent by controlling a weight percent of individual ingredient within operable or optimal Weight percentages indicated in the tables.

ATABLE 1 Operable composition of sintered body (mol. percent) ZnO: Additive 9995-900 0.05-10 A1203 99.95-90.0 0.05-10 Fe2O3 9995-90.() ..0.0510 Bi203 99.95-90.0 0.05-10 MgO 9995-90.() 0.05-10 CaO 9995-90.() 0.05-10 NiO 9995-900` 0.05-10 C0202 99.95-90.0 0.05-10 Nb2O5 99.95-900 0.05-10 Ta2O5 9995-900 0.05-10 ZrO2 9995-900 0.05-10 W03 9995-900 0.05-10 CdO 99.95-900 0.05-10 orzo,

Optimal composition of sintered body (mol. percent) ZnO: Additive 999-980 (Ll-2.0 Fe203 999-980 (Ll-2.0 Bi203 999-98.() 0.1-2.0 MgO 999-98.() 0.1-2.0 CaO 99.9-98.0 0.1-2.0 NiO TABLE 2 [Operable composition of electrode (wt. percent)] Ag PbO SiOz B203 BizOa CaO CuO 7300. 5 "d'e'-'z'i"'lif0bilis""'l'm'fi""lf" [Optimal composition of electrode (wt. percenm Ag PbO SiOz B203 BizOg CaO CuO Composition o( sintered body (mol. percent) Composition of electrode (wt. percent) ZnO Additive PbO SiOz B203 BigO; CaO C110 tageous compositions for saidl silver electrode Iwill be described hereinafter.

' The aforesaid electrode in ohmic contact can be of an electroless plated or electrolytic plated film of Ag, Cu, Ni, Zn, or Sn, a vacuum evaporated film of Al, Zn, Sn or In or a metallized film of Cu, Sn, Zn or Al in accordance with the prior well known technique.

Table 1 shows operable and optimal compositions of sintered body 1 for producing a non-linear resistor having a high n-value and a high stability with temperature, humidity and electric load.

Table 2 shows operable and optimal compositions of silver electrode after heating for curing in order to produce the novel non-linear resistors in accordance with the invention.

Table 3 shows optimal combinations of sintered body and silver electrode for producing non-linear resistors having a C-value lower than 6 at a given current of 100 ma., a high n-value and a high stability with temperature, humidity and electric load.

In Tables 2 and 3, indicated in the specification, a sum of the weight percents of all ingredients should be 100 1. 2-17 0. 1-6. 0 0. OOG-6. U 0-2. 0 0-2. 0

The sintered body can be prepared by a per se well known ceramic technique. The starting materials in the compositions defined in Table 1 are mixed in a wet mill so as to produce homogeneous mixtures. The mixtures are `dried and pressed in a mold into desired shapes at a pressure from kg./cm.2 to 1000 kg./cm.2. The pressed bodies are sintered in air at 1250 C. to .14S0 C. for 1 to 3 hours, and then furnace-cooled to room tempera- Dure (about 15 to about 30 C.). The pressed bodies are preferably sintered in non-oxidizing atmosphere such as nitrogen and argon when it is desired to reduce the electrical resistivity. The electrical resistivity also can be reduced by air-quenching from the sintering temperature to room temperature even when the pressed bodies are fired in air.

The mixtures can be preliminarily lcalcined at 700 to 1000 C. and pulverized for easy fabrication in the subsequent pressing step. The mixture to be pressed can be admixed with a suitable binder Isuch as water, polyvinyl alcohol, etc.

The sintered body is lapped at one surface, by abrasive powder such as silicon carbide, alumina, etc., or etched by a solution such as hydrochloric acid, sodium hydroxide, ammonium chloride, etc.

The sintered bodies are coated at -another lsurface without surface treatment or at the two opposed surfaces thereof by a silver electrodeA paint in a per se conven tional manner such as by a spray method, screen printing method or brushing method. It is necessary that the silver electrode paint have a solid ingredient composition as defined in Tables 2 and 3 after it is red at 100 C. t0 850 C. in air. Solid ingredients having compositions dellined in Tables 2 and 3 can be prepared in a per se conventional manner by mixing commercially available powders with organic resin such as epoxy, vinyl and phenol resin in an organic solvent such as butyl acetate, toluene or the like so as to produce silver electrode paints.

The silver powder can be in the form of metallic silver, or in the form of silver carbonate or silver oxide, or in any other form which, on firing at the temperature ernployed, will be converted to metallic silver. Therefore, the term silver as used throughout this specication and the claims appended hereto in connection with the silver composition before it is fired, is meant to include silver in any form which in firing will be converted to metallic silver. The viscosity of the resultant silver electrode paints can be controlled by the amounts of resin and solvent. Particle size of `solid ingredients also are required to be controlled in the range of 0.1/1l -to 5p.

-When only the surface without surface treatment is provided with the non-ohmic silver paint electrode, the one surface lapped or etched is provided with an oh'mic electrode such as an electroless plated `or electrolytic plated lilm of Ag, Cu, Ni, Zn or Sn, a vacuum evaporated film of Al, Zn, Sn or In or a metallizedlm of Cu, Sn, Zn or Al in accordance with the per se well known technique.

Lead wires can be applied to the so produced electrodes in a per se conventional manner Iby using conventional solder having a low melting point. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent for connecting the lead silver electrode paint be cured by heating in an oxidizing atmosphere such as air and oxygen and cooled to room temperature in the oxidizing atmosphere.

The following examples are given as illustrative of the presently-preferred method 0f proceeding according ito the present invention; however, it is not intended that the scope of said invention be limited to the specific examples.

EXAMPLE 4v1 Starting material according to Table 4 is mixed in a wet mill for 5 hours.

The mixture is dried and pressed in a mold into discs of 13 mm. in diameter and 2.5 mm. in thickness at a pressure of 340 kg./cm.2.

The pressed discs are sintered in air at 1350 C. for 1 hour, and then quenched to room temperature I(about 15 to about 30 C.). The sintered discs are divided into three groups with respect to surface treatment: The first group is not lapped at both surfaces. The -second group is lapped at both surfaces with silicon carbide in a particle size of 600 meshes, and the third group is lapped at one surface by silicon carbide in a particle size of 600 meshes. An electrical resistance measurement in |the aforesaid manner indictates that the surface layer of the disc as sintered has a higher electrical resistance than the inter-lor.

All sintered discs are coated at both surfaces with a silver electrode paint by 4a conventional brushing method after surface treatment. The silver electrode paint employed has the solid ingredient composition according to Table 5 and is prepared by mixing with vinyl resin in amyl acetate. The coated discs are tired at 500 C. for 30 minutes in air.

Lead wires are attached to the silver electrodes by means of silver paint. The electric characteristics of the resultant resistor and of other similary prepared resistors are shown in Table 4.

It will be readily understood that thesurface .treatment has a great effect on the electrical non-linearity of the wires to the sllver electrodes. Non-linear resistors accordresultant non-linear resistors.

TABLE 4 Starting materials (mol. percent) Electrical characteristics of resultant resistors Unlapped Both surfaces lapped One surface lapped ZnO Addiitves C at 100 ma. 'n C at 100 ma. n C at 100 ma. 'n

4. 24 2. 8 1. 91 3. 6 2. 10 5. 8 99.0 8. 68 3. 3 4. 21 4. 2 4. 68 7. 8 99.0 1.0 CaO 8.97 2.8 3.85 3.9 4.25 7. 2 99.0 1.0 NiO 6. 77 3. 1 2. 99 3. 7 3. 20 6. 8 99,5 0.5 C0203 5. 30 2. 9 2. 58 3. 7 2. 74 6. 6 99.5- 0.5 NbzO5 5. 25 2. 9 2. 56 3. 7 2. 74 5. 5 99.5- 0.5 TagO5 6. 19 2. 5 2. 84 3. 7 3. 12 5. 8 99.0 1.0 ZrOz 7. 84 3. 0 3.67 3. 9 4. 01 5. 4 99.0 1.0WO3 4.25 2.8 2.15 3.5 2.30 i 4.6 99.0 1.0 CdO 4. 43 2. 7 2. 11 3.2 2. 4l 4. 0 99.5 0.5 CrzOx 7. 19 3. 2 3. 49 3. 7 3. 88 4. 7

ing to this invention have a high stability to temperature and in the load life test, which is carried lout at 70 C. at a rating powder for 500 hours. The n-value `and C-v-alue do not change remarkably after heating cycles and load life test. It is preferable for achieving a high stability to humidity that the resultant non-linear resistors are embedded in a humidity proof resin such as epoxy resin and phenol resin in a per se well known manner.

According to the invention, it has been discovered that the cur-ing method of the applied silver electrode paint has a great effect on the ndvalue of the resultant nonlinear resistors. The n-value will not be -optimal when the applied silver electrode paint is heated -in a non-oxidizing atmosphere such as nitrogen and hydrogen for curing. It

EXAMPLE 2 Sintered.- discs prepared in the same manner as that in Example 1 are divided into three groups with respect to surface treatment. A disc of the first group is not lapped at both surfaces and is provided, at one surface, with a non-ohmic silver paint electrode similarly to the Example 1 and, at another surface, with an ohmic nickel electrode electrolytically plated. A disc of the second group is lapped at both surfaces and is provided, at one surface with a non-ohmic silver paint electrode and, at the other surface, with an ohmic nickel electrode electrolytically plated. A disc of the third group is lapped at one surface and is provided, at the unlapped surface, with a non-ohmic silver paint electrode and, at the lapped suris necessary for obtaining a high n-value that the applied face, with an ohmic nickel electrode electrolytically 7 plated. The 4electrical characteristics of the resultant resistors of these three groups are shown in Table 5.

It will be obvious that the non-linear resistors of the third group have higher n-value than those of the first and Second groups.

paint electrode in the same manner as that of Example 3 and another surface with the surface treatment is provided with an ohmic nickel electrode electrolytically plated in a per se well known method. The resultant non-linear resistor has a C-value of 0.8 at 100 ma. and an n-value of 20.

TABLE 5 Starting materials (mol. percent) Electrical characteristics of resultant resistors Unlapped Both surfaces lapped One surface lapped Additives C at 100 ma. n C at 100 ma. n C at 100 ma. n

0.5 .A1203 0. 80 7. 1 0. 50 9. 2 0. 55 14. 3 0.05 Fea 0. 93 8. 5 0. 70 12. 0 0. 73 20. 4

1 0 FezOa 0. 94 8. 9 0. 65 13. 0 0. 71 24.8 10.0 FezO 0. 99 3. 1 0. 75 3. 4 0.79 8. 3 0.5 131203 0. 90 5. 4 0. 65 6. 5 0.69 12. 7 1.0 MgO 0. 66. 4. 6 0.62 6. 0 0. 64 12. 1 1.0 CaO 0. 69 5. 7 0.66 7. 2 0. 67 13.6 1.0 NiO 0.72 4. 8 0.56 5. 5 0. 59 9.6 0.5 C0203 0. 75 6. 6 0. 68 7. 2 0. 72 12.3 0.5 Nb205 0.63 5. 6 0.52 6. 8 0. 58 9.8 0.5 'IagrOls 0. 74 6. 4 0. 65 8. 7 0.70 12. 4 1.0 ZrOz 0.81 7. 1 0. 73 8. 3 0.75 12. 1 1.0 W03 0. 77 4. 9 0.68 7. 6 0. 71 11. 3 1.0 CdO 0. 76 6.0 0. 68 7. 5 0.71 11.5 0.5 CrzOsl 0. 79 6. 2 0.68 8. 6 0. 72 12.6

EXAMPLE 3 EXAMPLE 5 Sintered discs in a composition of 99.5 mol. percent of zinc oxide and 0.5 mol. percent of iron oxide are prepared in the same manner as that in Example 1. Said sintered discs are divided into three groups with respect to surface treatment. The first group is etched, at both surfaces, :by an aqueous solution of 6 N HC1 for 10 minutes. The second group is not etched at both surfaces and the third group is etched, at one surface, by said aqueous solution of 6 N HCl for 10 minutes. Each of these discs is provided with, at both surfaces, silver paint electrodes defined by Table 6 similarly to the foregoing Examples.

, The load life test is carried out at 70 C. ambient tem- -pe'rature at 1 watt rating power for 500 and 2000 hours. The heating cycle test is carried out by repeating 5 times the cycle in which said resistors are kept at 85 C. am bient temperature for 30 minutes, cooled rapidly to 20 C. and then kept at such temperature for 30 minutes.

After heatin-g'cycles and load life test, the change rates of the C-value and n-value are shown in Table 8. It will be understood that the asymmetrical surface treatment has a great effect on the stability of the resultant resistors.

TABLE 8 Change rates of electrical characteristics (percent) Unlapped Both surfaces lapped One surface lapped C (at C (at C (at Test 100 ma.) n 100 ma.) 'n 100 ma.) 'n

Both electrodes silver. Heating cycle 2, 1 1 g Load life test 500 hours..- 0. 7 1. 3 1 Load life test 2,000 hours. 1. 0 2. 7 1. 9 16. 4 0.6 1. 5 One electrode silver, Heating cycle 0. 4 0. 5 1. 4 2. 7 0. 4 0. 4 another electrode nickel. Load life test 500 hours 0, 1 ...0, 3 0 9 3, 0 Q 1 O 2 Load life test 2,000 hours. 0. 2 -0. 5 0. 9 6. 2 0. 2 0. 5

TABLE 6 What is claimed is:

[Composition oisilver electrode (Wtpercentl] 1. A non-linear resistor comprising a sintered wafer of Ag pbo S102 B203 Cao zinc oxide having opposite surfaces one of which has 90 7 0 2 0 o 7 0 3 an electrlcal resistance higher than that of the other sur- Table 7 listing the electrical characteristics of the resultaut non-linear resistors shows that the non-linear re sistors of the third group are superior to those of the first and second groups.

A sintered disc in the same composition as that of Example 3 is subjected to surface treatment in the same manner as that of the third group of Example 3. One surface without the surface treatment is provided with a silver face and an electrode Iapplied to each of said opposite surfaces, at least one of said two electrodes being in nonohmc contact with said one surface having a higher electrical resistance.

2. A non-linear resistor as defined by claim 1, wherein said one electrode in non-ohmic contact comprises a silver paint electrode.

3. A non-linear resistor as defined by claim 1, wherein the other of said electrodes is in ohmic contact with said other surface having a lower electrical resistance.

4. A non-linear resistor as defined by claim 1, wherein said two electrodes comprise silver paint electrodes in non-oh-mic contact with said opposite surfaces.

5. A non-linear resistor as defined by claim 3, wherein said other electrode in ohmic contact comprises an electrolytic plated or electroless plated film of Ag, Cu, Ni, Zn or Su, a Vacuum evaporated film of Al, Sn, Zn or In or metallized film of Cu, Sn, Zn or Al.

6. In a method for making a non-linear resistor of a sintered wafer of zinc oxide having electrodes applied to opposite surfaces thereof, the improvement Which consists of sinteringa pressed body in a wafer form at 1100 C. to 1500 C. in air, cooling the sintered wafer to room temperature, subjecting the coo-led wafer, "at one surface, to a surface-treatment for removal of'fsurrface layer, whereby said one surface has an electrical resistance different from that of another surface.

7. The improvement in making .a non-linear resistor defined by claim 6, wherein said cooled wafer is lapped, at one surface, with a lapping powder. A

8. The improvement in making a non-linear resistor defined by claim 6, wherein said cooled wafer is etched, at one surface, with an etching reagent.

9. A non-linear resistor according to claim 3, wherein said sintered wafer consists of zinc oxide (ZnO).

10. A non-linear resistor according to claim 3; wherein said sintered wafer comprises 99.95 to 90 mol. percent of zinc oxide (XnO) and 0.05 to 10.0 mol percent of at least one oxide selected from the group consisting of iron oxide (FezOa), aluminum oxide (A1203), bismuth oxide (Bi203),`magnesium oxide (MgO), calcium oxide (CaO), nickel oxide (NiO), cobalt oxide (C0203), niobium oxide (Nb205), tantalurn oxide (Ta205), zir- 10 conium oxide (ZrOz) tungsten oxide (W03), cadmium oxide (CdO), and chromium oxide (Cr203).

11. A non-linear resistor according to claim 2, wherein said silver paint electrodes have a composition of 100 wt. percent of silver.

12. A non-linear resistor yaccording to claim 2, wherein said silver electrodes have the composition comprising 70 to 99.5 wt. percent of silver, 0.25 to 27 wt. percent of lead oxide (PbO), 0.02 to 15 wt. percent of silicon dioxide (SiOz), 0.01 to l5 wt. percent of boron oxide B203), 0 to 6.0 wt. percent of bismuth oxide (BiZOa), 0 to 6.0 wt. percent of calcium oxide (CaO) .and 0 to 6.0 wt. percent of cupricl oxide (CuO).

References Cited UNITED STATES PATENTS 2,027,277 1/ 1936 Hahann 338-20 2,720,573 10/ 1955 LundquiSt 317--238 2,887,632 5/1959 Dalton 252--512 X RODNEY D. BENNETT, I R., Primary Examiner H. C. WAMSLEY, Assistant Examiner U.S. C1. X.R. 

